Process for the electrodeposition of zinc



Patented Jan. 30, 1951 PROCESS FGR THE ELECTRODEPOSITION OF ZINC Robert P. Yeck, Westfield, and Yurii E. Lebedefi,

Metuchen, N. 3., assignors to American Smelting and Refining Company, New York, N. Y., a corporation of New Jersey No Drawing. Application October 26, 1948, Serial No. 56,665

3 Claims.

This invention relates to the art of producing electrolytic zinc and has as its principal object, the definite improvement of that art as heretofore conventionally practiced.

What may be termed conventional practice in the electrodeposition of zinc comprises electrolyzing an acid electrolyte, the major nonaqueous constituents of which are substantial quantities of sulphuric acid and of zinc sulphate, employing insoluble anodes, whereby zinc from the electrolyte is deposited upon cathode blanks of aluminum or the like and from which the zinc cathode deposit is subsequently stripped. The insoluble anodes consist, at least in major portion, of lead, e. g., the well known lead-silver anode. A judicious use of manganese in the process tends to lessen lead contamination of the zinc cathode deposit; and so-called addition agents are also normally incorporated in the electrolyte for such purposes as to suppress acid spray or mist and to improve the qua ity of the cathode deposit.

One of the major problems in the production of electrolytic zinc, and one which is as old as the art itself, has been that of obtaining a zinc cathode deposit of desired low lead content. Manganese dioxide is commonly employed in the art,

as is also air, as an oxid zing agent for removing iron from the zinc sulphate solutions going to which is insufficient to markedly affect the acidity of the electrolyte but which is sumcient, upon electrolysis of the latten to result in the production of a zinc cathode deposit of substantially lower lead content that that of a similar zinc cathode deposit produced under conditions that are otherwise identical except for the addition of such amount of strontium.

The efliciency of the process of the invention in producing a cathode zinc deposit of extremelly low lead content will be at once apparent from a comparison of the results obtained in the operation of a pilot cell, without and with strontium, and summarized below in Tables I and II, respectively. Such cell was equipped with leadsilver anodes (1% Ag, balance Pb) and an aluminum cathode and was operated in all ten runs at a current density of approximately '70 amps./sq.ft. of cathode surface, with the electrolyte at a temperature of C. Acid content of make up the electrolyte. Further, the proper use i and control of manganese in the electrolytic cells has been found to be of substantial benefit in curbing the lead content of the cathode zinc deposit; But while manganese is very helpful in keeping down the lead content of the zinc cathode deposit, it has also its limitations in that regard and fortunate indeed is the electrolytic zinc plant which experiences no difficulty in having all of its metal meet the special high grade zinc specification.

Now, by reason of the present invention, it can be fairly stated that there is no longer any problem of consequence in readily producing electrolytic zinc in which the lead contaminant falls far below the maximum permitted by the special high grade specification-in fact, the invention permits of the production of electrolytic zinc of such low lead content as to meet the most exacting customer requirements encountered to date in that regard. Briefly, the present invention achieves this remarkably improved result by incorporating an effective amount of strontium in the electrolyte and conducting the electrolysis along the lines of the conventional practice heretofore described; an effective amount of strontium being defined as any quantitythereoi the electrolyte was 190-200 grms. H2SO4 per liter. The essential difference between the two groups of runs was that Nos. 201-205 inclusive were conducted in the absence of strontium while in Nos. 216-220 inclusive the electrolysis was performed in the presence of strontium which was added to the electrolyte in the form of a strontium carbonate slurry at the rate of 45 lbs. SICOs per ton of cathode zinc produced.

TabZe I (no Sr) Per cent Electrolyte Clnfrlrent b in Run Cathode ency, per

De osit cent p Inga/l. gins/l. gins/l.

0. 42 0. 9 71 90. 2 (l. 1. 0 67 88. 3 0. 57 0. 8 64 90. i 0. 61 1. 2 68 90. 6 0. 78 l. 2 66 91. 6

Table II (with Sr) gbw Electrolyte I Ougent In E 0? Run Cathode Pb Mn 7p ency, poi

- i a Deposlt mgs./l. gins/l. guns [1. (km

0. 13 l. 3 67 91, 2 0. 12 0. 9 67 91. 5 0. 11 l. O 58 91. 4 0. 09 1. l 66 92. 0 0. 10 1. 2 5G 90. O

For the five runs reported in Table I, using manganese in accordance with known practice to control the lead, the average lead content of the'plants operations was initiated.

3 the cathode zinc deposits was 0.00534%; whereas, in the case of the five runs reported in Table II, in which the electrolysis was conducted in the presence of strontium in accordance with the invention, the average lead content of the zinc cathode deposits was only 0.0006l%a decrease in lead content of the cathode zinc of approximately 88%.

As previously mentioned, strontium was added to the electrolyte in the form of a slurry of SrCO, in the runs reported in Table II. While this is the presently preferred mode for adding" the strontium to the electrolyte, the invention may be practiced in other manners. Thus, the in-' vention has been practiced in the pilot cell afore-- said with fairly good results by adding strontium to the electrolyte in the form of SrClz and SLSQi; respectively, and other ways of performing the electrolysis in the presence of strontium will no doubt occur to those skilled in the art. Of the alkaline earth metalsfbarium has been'found capable of' keeping down the lead content of the cathode zinc deposit in a manner similar to strontium though results to date indicate that it is somewhat less efficacious than strontium in this regard. However, with that'qu'alification, barium is to be considered the equivalent of strontium in the present description and claims. In addition 'to inhibiting lead contamination of the cathode zinc deposit, there are indications that "conducting the"el'ectrolysis in the presence of: strontium may afford other advantages, such as performing the electrodepositiontvlth the electrolyte at considerably higher temperatures, increased 'current'efficiency, etc. It is also to'be' noted that in neither pilot nor commercial plant operations using the invention, has any toxic or deleterious effect on the electrolytic zinc process been detected or observed.

"There is probably no better illustration of the advance in the art wrought by the present inven ti'on, 'th'an'the facts attending its introduction into'commercial use in the electrolytic zinc plant at" Corpus Christi, Texas." In'that'plant', which has a yearly capacity of 30,000 tons of slab zinc; the cells are equipped with standard lead-silver anodes (99% Pb,1Ag) and aluminum cathodes (commercially pure *Al containing not more than 0.07% Cu).

During the first full, halfr'mfinth period immediately preceeding the introduction of the invention, and which may be taken as fairly typical oftheplants practice, the following average con ditions obtained: temperature of electrolyte- 38-40 0.; current density-'56 amps./sq,ft. of cathode surface; current eiiiciency85.2%. The electrolyte itself averaged 192 grms./l. H2804; 65 grms.'/l. Zn; 3.7 grms./l. Mn; and 1.6 mgs/l, Pb. The addition agents usedwere: 0.49 lb. gum. arabic, 0.31 lb, cresylic acid, and 4.7 lbs. sodium silicate-all per ton of cathode zinc produced. The average lead content of the'c'athode zinc produced during the period was L0037%."

For the two days immediately preceding the introduction of the invention into the plant, the lead content of the zinc cathodes was 0.0041% and 0.004270, respectively. On the next morning the introduction of strontium into the electrolyte circuitsf was commenced at a rate to yield 2.5 lbs'. $11003 per ton of cathode zinc produced. The. beneficial effect was apparentp'ractically at once as will be seen from Table III which sets forth the results for the seven days next succeeding that on which introduction of the invention into Striking as these results are, they are now Viewed as routine in the operation of the Corpus Christi plantf' Thus, during the third full 15- day operating period after the invention was introduced' into the plant, the lead content of the cathode zinc produced averaged only 0.0015%- the only material change in the plant, operation, from what it was, prior, to. the invention, bein that the electrolysis was. conducted in the pres; ence of strontium by adding a'slurry of SrCQe, to the electrolyte at the. rate, of only 2.2 lbs; per ton'of cathode zinc produced.

What is claimed is:

'1'. In the art of electrodepositing zinc from an'acid electrolyte, the major ricnaqueous. constituents ofwhich are substantial; quantities of; sulphuric acid and of zinc, sulphate, using. in,- soluble anodes of which atleast; the major,p o r; tion is lead, that improvement, for enhancing, the. purity of the zinc, cathode deposit which consists in adding strontium tof the electrolyte in amount insufficient to markedly afigctl the acidity ofthe latter but sufiicient to lessen. the amount iead: entering into the, zinc cathode deposit during, electrolysis, andv electrodepositing, zinc, from said el r lyt h r n aim sa indca .cdedci posit of lower lead content than would result but for. such addition of; strontium.

2. The'process according to. claim, L inwhich the strontium is added to the electrolyte, in. the. form'of a slurry of strontium, carbonate.

' 3. The. process for producing electrolytic zinc. which consists in, providing. an. acid; electrolyte. comprising. sulphuric acid. andjzinc sulphate. as the major nonaqueous constituents and; mane. ganese as a. minor (constituent, incorporating; an. efiective amount of strontiumcinjthe,electrolyte; and electrolyzing the electr lyte u in n cadrsilven anodes, thereby attaining. a, cathodedeposit, off; zinc of substantially lower, lead; content, th Would, prevail in the j absence, of, said strontium addition.

ROBERT- P. YECK.

YURHrEi LEEEDEFF.

REFERENCES, CITED) ha llQW -i ref r ce 3 9 iq i le. UNITEDISE'ALTESGPATEMSJ Number Name Date 1,163,286 V adr'ien Dec. 7,1915 1,163,911" Dec; 14; 1915; 1,167,701" can. 1 1, 1 916} FOREIGN- RATEIIZES Number Country Date,

8,467- Great Britain jdfussz 273,420 Great 'Britain @1192?- 24,682 Germany 26,091 Germany 

1. IN THE ART OF ELECTRODEPOSITING ZINC FROM AN ACID ELECTROLYTE, THE MAJOR NONAQUEOUS CONSTITUENTS OF WHICH ARE SUBSTANTIAL QUANTITIES OF SULPHURIC ACID AND OF ZINC SULPHATE, USING INSOLUBLE ANODES OF WHICH AT LEAST THE MAJOR PORTION IS LEAD, THAT IMPROVEMENT FOR ENHANCING THE PURITY OF THE ZINC CATHODE DEPOSIT WHICH CONSISTS IN ADDING STRONTIUM TO THE ELECTROLYTE IN AMOUNT INSUFFICIENT TO MARKEDLY AFFECT THE ACIDITY OF THE LATTER BUT SUFFICIENT TO LESSEN THE AMOUNT OF LEAD ENTERING INTO THE ZINC CATHODE DEPOSIT DURING ELECTROLYSIS, AND ELECTRODEPOSITING ZINC FROM SAID ELECTROLYTE, THEREBY OBTAINING A ZINC CATHODE DEPOSIT OF LOWER LEAD CONTENT THAN WOULD RESULT BUT FOR SUCH ADDITION OF STRENTIUM. 